In view of the serious energy problems, solar energy represents one form of energy which has received a great deal of attention. Various approaches have been taken by those in the art to tap this source of energy. Such approaches have included the fabrication of solar cells. In general, solar cells include semiconductors disposed intermediate a pair of electrical contacts. One such form which has been considered is the utilization of amorphous silicon as the semiconductor material.
Amorphous silicon has many properties which would make it a desirable material for photovoltaic devices. For example, it has a bandgap (1.5-2.0 eV) which lies in the range needed for high efficiency. Additionally, amorphous silicon is easy to deposit. Further it has absorption coefficient. Hence, a thin film can absorb most of the light. Moreover, its electronic properties can be changed by doping with n and p type dopants. Despite these attractive properties, present day amorphous silicon devices have not achieved high efficiencies. Photovoltaic conversion efficiency, for example, has been limited to an about 6% range.
To date, the highest efficiencies with such amorphous silicon solar cells has been obtained by using Metal-Insulator-Semiconductors (MIS) or Schottky diodes. However, both MIS and Schottky diodes suffer from various disadvantages. In this respect, the necessity of using a metal layer reduces light transmission into the semiconductor. Additionally,, control of interface state densities is particularly difficult, and hence the cells tend to have non-reproducible or low outputs.
As an alternative to MIS and Schottky diodes, homojunction solar cells using amorphous silicon has also been studied. However, the results were poor, partly because of excessive recombination at the interface between p and n (with i) regions of the junction.